In the workgroup’s “Actions” menu choose “Group Parameters”:
In the “Edit group” window “Description” section - click “move to archive”:
After you’ve moved the group to the archive it will no longer be listed among user’s workgroups. The archive workgroups can be found in All workgroups > Archive (or with the help of Search tool):
Please note that you can always move the workroup back from archive, to do that you need to disable "moove to archive" option in the workgroup parameters.